U.S. patent number 7,383,917 [Application Number 11/088,180] was granted by the patent office on 2008-06-10 for running stability control device for vehicle based upon longitudinal forces of wheels.
This patent grant is currently assigned to Toyota Jidosha Kabushiki Kaisha. Invention is credited to Kenji Asano.
United States Patent |
7,383,917 |
Asano |
June 10, 2008 |
Running stability control device for vehicle based upon
longitudinal forces of wheels
Abstract
A running stability control device for a vehicle having a
steering device capable of steering steered vehicle wheels
independently of a steering operation by a driver, estimating
longitudinal forces of respective vehicle wheels, computing a yaw
moment generated by a difference in the longitudinal forces of the
vehicle wheels at left and right sides of the vehicle, computing a
first compensation amount for a steering angle for decreasing the
yaw moment, computing lateral forces generated in the steered
vehicle wheels when they are steered for the first steering angle
compensation amount, computing a second compensation amount for the
steering angle for decreasing a sum of the lateral forces generated
in the front and rear vehicle wheels when the steered vehicle
wheels are steered for the first steering angle compensation
amount, and computing a final steering angle for the steered
vehicle wheels based upon the first and second steering angle
compensation amounts.
Inventors: |
Asano; Kenji (Toyota,
JP) |
Assignee: |
Toyota Jidosha Kabushiki Kaisha
(Toyota, JP)
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Family
ID: |
35034272 |
Appl.
No.: |
11/088,180 |
Filed: |
March 24, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050247510 A1 |
Nov 10, 2005 |
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Foreign Application Priority Data
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Mar 26, 2004 [JP] |
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2004-091512 |
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Current U.S.
Class: |
180/446; 180/410;
701/41; 701/42 |
Current CPC
Class: |
B60T
8/1755 (20130101); B62D 6/003 (20130101); B62D
6/04 (20130101) |
Current International
Class: |
B62D
5/04 (20060101) |
Field of
Search: |
;180/410,446
;701/41,42 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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38 26 982 |
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Feb 1989 |
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DE |
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40 38 079 |
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Jun 1992 |
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DE |
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101 50 605 |
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Apr 2003 |
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DE |
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1 293 412 |
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Mar 2003 |
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EP |
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A 7-81600 |
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Mar 1995 |
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JP |
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B2 2540742 |
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Jul 1996 |
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JP |
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A 2002-302059 |
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Oct 2002 |
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JP |
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Primary Examiner: Morris; Lesley D.
Assistant Examiner: Yeagley; Daniel
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
The invention claimed is:
1. A running stability control device for a vehicle having a
steering device capable of steering steered vehicle wheels
independently of a steering operation by a driver, the running
stability control device comprising a computing device for
estimating longitudinal forces of respective vehicle wheels,
computing a yaw moment generated by a difference in the
longitudinal forces of the vehicle wheels at left and right sides
of the vehicle, computing a first compensation amount for a
steering angle for steering the steered vehicle wheels to decrease
the yaw moment due to the difference in the longitudinal forces of
the left and right side vehicle wheels, computing lateral forces
generated in the vehicle wheels at front and rear of the vehicle
when the steered vehicle wheels are steered for the first steering
angle compensation amount, computing a second compensation amount
for the steering angle of the steered vehicle wheels for decreasing
a sum of the lateral forces generated in the front and rear vehicle
wheels when the steered vehicle wheels are steered for the first
steering angle compensation amount, and computing a final steering
angle for the steered vehicle wheels based upon the first and
second steering angle compensation amounts, and operating the
steering device for steering the steered vehicle wheels so that the
steered vehicle wheels are steered for the final steering
angle.
2. A running stability control device according to claim 1, wherein
the computing device estimates a slip angle of the vehicle when the
steered vehicle wheels are steered for the first steering angle
compensation amount, sets out a yaw angle of the vehicle to be an
angle laterally opposite to the slip angle, and computes the second
steering angle modification amount as a modification steering angle
for the steered vehicle wheels for attaining the yaw angle.
3. A running stability control device according to claim 2, wherein
the yaw angle of the vehicle is set out to be not larger than the
slip angle of the vehicle.
4. A running stability control device according to claim 2, wherein
the yaw angle .beta.t of the vehicle is set out according to the
following formula:
.beta..times..times..times..delta..times..times..times..gamma..t-
imes. ##EQU00010## where V, .gamma., .delta.f, Cpf, Cpr, Lf and Lr
are vehicle speed, yaw rate of the vehicle, steering angle of the
front vehicle wheels, cornering power of the front vehicle wheels,
cornering power of the rear vehicle wheels, longitudinal distance
between the mass center of the vehicle and the axis of the front
vehicle wheels and longitudinal distance between the mass center of
the vehicle and the axis of the rear vehicle wheels,
respectively.
5. A running stability control device according to claim 1, wherein
the computing device estimates a temporary target steering angle
based upon the steering operation of the driver, and computes a
final steering angle by modifying the temporary target steering
angle by the first and second steering angle compensation amounts.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a running stability control device
for a vehicle, and more particularly to a running stability control
device for a vehicle for controlling the running stability of the
vehicle by controlling the steering angle of the steered vehicle
wheels of the vehicle.
2. Description of the Prior Art
As a running stability control device for a vehicle such as an
automobile, there is known a device to steer the steered vehicle
wheels by a steering device so as to cancel the yaw moment acting
at the vehicle by a difference in the braking forces applied to the
left and right vehicle wheels during an anti-skid control of the
vehicle equipped with the steering device by which the steering
angle of the steered vehicle wheels can be changed independently of
the steering operation by the driver as described, for example, in
the publication of Japanese Patent No. 2540742.
By such a running stability control device, when a yaw moment is
applied to the vehicle due to a difference in the braking forces of
the left and right vehicle wheels during an anti-skid control, the
yaw moment is cancelled by the steering of the steered vehicle
wheels, so that thereby the running of the vehicle in a wrong
direction is prevented, and the running stability of the vehicle is
improved without sacrificing the deceleration performance or the
distance traversed by the vehicle which would occur when the
braking force at a vehicle wheel serving at a side of the road of a
higher friction coefficient is decreased.
SUMMARY OF THE INVENTION
When the yaw moment due to a difference in the braking forces of
the left and right vehicle wheels is cancelled by a steering of the
steered vehicle wheels, the steering device is generally steered
toward a side of the road of a lower friction coefficient, whereby
the vehicle is shifted laterally toward the lower friction
coefficient side of the road by the steering action, whereby the
running course of the vehicle is distorted toward the lower
friction coefficient side of the road relative to the due running
course. On the contrary, when the steering device is operated to
steer the vehicle toward a side of a higher friction coefficient of
the road to cancel a yaw moment applied to the vehicle by a
difference in the driving forces of the left and right vehicle
wheels when such a difference has occurred during an acceleration
of the vehicle, the vehicle is shifted laterally toward the higher
friction coefficient side of the road by a lateral force generated
by such a steering control, whereby the running course of the
vehicle deviates from the due course toward the higher friction
coefficient side.
In view of the above-mentioned problems in the conventional running
stability control device adapted to steer the steered vehicle
wheels for canceling the yaw moment applied to the vehicle by a
difference in the driving/braking forces of the left and right
vehicle wheels, it is a primary object of the present invention to
provide a running stability control device for a vehicle by which a
deviation of the vehicle from the normal running course due to a
difference in the driving/braking force of the left and right
vehicle wheels is effectively prevented so as to definitely improve
the stability of the running of the vehicle, thereby effectively
decreasing a lateral shifting of the vehicle from the normal
running course as induced by a lateral force generated in the
steered vehicle wheels due to the operation of the steering
device.
According to the present invention, the above-mentioned object is
accomplished by a running stability control device for a vehicle
having a steering device capable of steering steered vehicle wheels
independently of a steering operation by a driver, the running
stability control device comprising a computing device for
estimating longitudinal forces of respective vehicle wheels,
computing a yaw moment generated by a difference in the
longitudinal forces of the vehicle wheels at left and right sides
of the vehicle, computing a first compensation amount for a
steering angle for steering the steered vehicle wheels to decrease
the yaw moment due to the difference in the longitudinal forces of
the left and right side vehicle wheels, computing lateral forces
generated in the vehicle wheels at front and rear of the vehicle
when the steered vehicle wheels are steered for the first steering
angle compensation amount, computing a second compensation amount
for the steering angle of the steered vehicle wheels for decreasing
a sum of the lateral forces generated in the front and rear vehicle
wheels when the steered vehicle wheels are steered for the first
steering angle compensation amount, and computing a final steering
angle for the steered vehicle wheels based upon the first and
second steering angle compensation amounts, and operating the
steering device for steering the steered vehicle wheels so that the
steered vehicle wheels are steered for the final steering
angle.
According to such a running stability control device, since the yaw
moment due to a difference in the longitudinal forces acting in the
left and right side vehicle wheels is computed, a first steering
angle compensation amount for the steered vehicle wheels is
computed for decreasing the yaw moment, a lateral force generated
in the front and rear vehicle wheels due to the steering of the
steered vehicle wheels for the first steering angle compensation
amount is computed, a second steering angle compensation amount for
the steered vehicle wheels is computed for decreasing the sum of
the lateral forces generated in the front and rear vehicle wheels,
and a final steering angle for the steered vehicle wheels is
computed based upon the first and second steering angle
compensation amounts, so that the steered vehicle wheels are
steered for the final steering angle, a deviation of the vehicle
from the normal course due to the difference in the driving/braking
forces in the left and right side vehicle wheels is effectively
prevented so as definitely improve the running stability of the
vehicle, while decreasing the overall lateral force of the vehicle,
thereby effectively decreasing a lateral deviation of the vehicle
from the normal running course due to the lateral force generated
in the vehicle wheels due to the steering control.
The running stability control device according to the present
invention may be so constructed that the computing device estimates
a slip angle of the vehicle when the steered vehicle wheels are
steered for the first steering angle compensation amount, sets out
a yaw angle of the vehicle to be an angle laterally opposite to the
slip angle, and computes the second steering angle modification
amount as a modification steering angle for the steered vehicle
wheels for attaining the yaw angle.
The yaw angle of the vehicle may set out to be not larger than the
slip angle of the vehicle.
More concretely, the yaw angle of the vehicle may be set out
according to the below-mentioned formula 18.
According to such a running stability control device, since the
slip angle of the vehicle due to the steering of the steered
vehicle wheels for the first steering angle compensation amount is
estimated, an angle of the vehicle laterally opposite to the slip
angle is set out as a yaw angle, and a second steering angle
compensation amount for the steering angle for the steered vehicle
wheels is computed to attain the target yaw angle, the lateral
forces generated in the front and rear vehicle wheels by the
steered vehicle wheels being steered for the first steering angle
compensation amount is correctly compensated by the second steering
angle compensation amount through the computation of the slip angle
of the vehicle.
The running stability control device according to the present
invention may be so constructed that the computing device estimates
a temporary target steering angle based upon the steering operation
of the driver, and computes a final target steering angle by
modifying the temporary target steering angle by the first and
second steering angle compensation amounts.
According to such a running stability control device, since a
temporary target steering angle based upon the steering operation
of the driver is modified by the first and second steering angle
compensation amounts to provide a final target steering angle for
the steered vehicle wheels, the vehicle runs correctly along a
course instructed by the steering operation of the driver
regardless of a difference in the longitudinal forces of the
respective vehicle wheels.
The running stability control device according to the present
invention may be so constructed that it is operated to execute the
driving/braking force control for stabilizing the running stability
of the vehicle when neither a spin suppress control nor a driftout
suppress control is being executed.
By denoting vehicle speed as V, yaw rate of the vehicle as .gamma.,
slip angle of the vehicle as .beta., steering angle of the front
vehicle wheels as .delta.f, mass of the vehicle as M, yaw inertial
moment of the vehicle as Iz, cornering power of the front vehicle
wheels as Cpf, cornering power of the rear vehicle wheels as Cpr,
longitudinal distance between the mass center of the vehicle and
the axis of the front vehicle wheels as Lf, longitudinal distance
between the mass center of the vehicle and the axis of the rear
vehicle wheels as Lr, yaw moment due to a difference in the
longitudinal forces of the left and right vehicle wheels as Mf,
change rate of the yaw rate .gamma. of the vehicle as .gamma.d,
change rate of the slip angle .beta. of the vehicle as .beta.d, and
a11, a12, a21, a22, b1, b2 and c2 as such amounts expressed by the
below-mentioned formulae 2-8, respectively, there is a relationship
such as expressed by the following formula 1.
.beta..times..times..gamma..times..times..times..times..times..times..tim-
es..times..times..times..function..beta..gamma..times..times..times..times-
..times..delta..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.
##EQU00001##
By denoting the Laplace operator as s, a transfer function
H.gamma..delta. from the front vehicle wheel steering angle
.delta.f(s) to the yaw rate .gamma.(s) of the vehicle and a
transfer function H.gamma..sub.M from the yaw moment Mf(s) due to a
difference in the longitudinal forces of the left and right vehicle
wheels to the yaw rate .gamma.(s) of the vehicle are respectively
expressed by the following formulae 9 and 10.
.gamma..times..times..delta..gamma..function..delta..times..times..functi-
on..times..times..times..times..times..times..times..times..times..times..-
times..times..times..times..times..times..times..times..times..times..time-
s..times..times..times..times..gamma..times..times..gamma..function..funct-
ion..times..times..times..times..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times.
##EQU00002##
According to the above formulae 9 and 10 there exists a
relationship such as shown by the following formula 11, from which
the steering angle .delta.f(s) of the front steered vehicle wheels
to cancel the yaw moment Mf(s) due to the difference in the
longitudinal forces of the left and right vehicle wheels is
obtained by the following formula 12.
H.sub..gamma..delta..delta.f(s)=H.sub..gamma.MMf(s) (11)
.delta..times..times..function..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..tim-
es..times..times..function. ##EQU00003##
From the above formula 12 the steering angle .delta.f of the
steered front vehicle wheels to cancel the yaw moment Mf due to the
difference in the longitudinal forces of the left and right vehicle
wheels is obtained as the following formula 13 as a static value
not considering a transient response for convenience.
.delta..times..times..function..times..times..times..times..times..times.-
.times..times..times..times..times..times..times..times..times..times..fun-
ction. ##EQU00004##
Therefore, the running stability control device may be so
constructed as to compute a first steering angle compensation
amount according to the above formula 12 or 13.
Further, the slip angle .alpha.f of the front vehicle wheels and
the slip angle .alpha.r of the rear vehicle wheels are computed by
the following formulae 14 and 15, respectively.
.alpha..times..times..beta..times..gamma..delta..times..times..alpha..tim-
es..times..beta..times..gamma. ##EQU00005##
Therefore, in order for the lateral force Fyf=Cpf.alpha.f generated
by the steered front vehicle wheels to balance with the lateral
force Fyr=Cpr.alpha.r of the rear vehicle wheel as shown in FIG. 4,
it should be such that Fyf+Fyr=0, i.e., the following formula 16 is
established.
.function..beta..times..gamma..delta..times..times..function..beta..times-
..gamma. ##EQU00006##
Therefore, the slip angle .beta. of the vehicle in the case that
the steering angle of the steered front vehicle wheels is
controlled to cancel the yaw moment Mf due to the difference in the
longitudinal forces between the left and right vehicle wheels is
expressed by the following formula 17.
.beta..times..delta..times..times..times..gamma..times.
##EQU00007##
Therefore, in order for the lateral force Cpf.alpha.f generated by
the front vehicle wheels due to the control of the steering angle
of the steered front vehicle wheels to be cancelled by the lateral
force Cpr.alpha.r of the rear vehicle wheels, the yaw angle .beta.t
of the vehicle is to be such that is opposite in the lateral
direction to the slip angle .beta. of the vehicle expressed by the
formula 17 and less than the slip angle .beta., more desirably, a
value expressed by the following formula 18.
.beta..times..times..beta..times..delta..times..times..times..gamma..time-
s. ##EQU00008##
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawing,
FIG. 1 is a diagrammatic view showing an embodiment of the
stability control device of the vehicle according to the present
invention applied to a four wheel drive vehicle equipped with a
semi-steer-by-wire type steering angle varying device as an
automatic steering device;
FIG. 2 is a flowchart showing the steering angle control of the
front left and front right vehicle wheels;
FIG. 3 is a map showing the performance of the steering gear ratio
Rg according to the vehicle speed V; and
FIG. 4 is a diagrammatical view showing the running condition of a
vehicle on a road in which the friction coefficient of the road is
different on the left and right sides of the road.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the following, the present invention will be described in more
detail with respect to a preferred embodiment thereof.
Referring to FIG. 1 showing an embodiment of the running stability
control device of the vehicle according to the present invention
applied to a rear driven vehicle equipped with a steering device
incorporating a steering angle varying device operating as an
automatic steering device in a schematic illustration, 10FL and
10FR are steered front left and front right vehicle wheels of the
vehicle 12, and 10RL and 10RR are driving rear left and rear right
vehicle wheels. The front left and front right vehicle wheels 10FL
and 10FR are steered according to the steering operation of a
steering wheel 14 by a driver via a rack-and-pinion type power
steering device 16, a rack bar 18 and tie rods 20L and 20R. The
steering wheel 14 is connected with a pinion shaft 30 of the power
steering device 16 via an upper steering shaft 22, a steering angle
varying device 24, a lower steering shaft 26 and a universal joint
28. In the shown embodiment, the steering angle varying device 24
includes an electric motor 32 having a housing 24A connected to the
lower end of the upper steering shaft 22 and a rotor 24B connected
to the upper end of the lower steering shaft 26.
The steering angle varying device 24 variably rotates the lower
steering shaft 26 relative to the upper steering shaft 22 so as to
variably change the ratio of the steering angle of the front left
and front right vehicle wheels 10FL and 10FR relative to the
steering rotation of the steering wheel 14 as controlled by a
steering control portion of an electric control device 34. The
steering angle varying device 24 normally rotates the lower
steering shaft 26 in a predetermined relationship relative to the
rotation of the upper steering shaft 22, but automatically changes
the rotation of the lower steering shaft 26 relative to the upper
steering shaft 22 according to its automatic control operation as
required.
When a failure has occurred in the steering angle varying device 24
such that the lower steering shaft 26 is not rotated relative to
the upper steering shaft 22, a locking means not shown in FIG. 1 is
actuated, so that the relative rotation of the housing 24A and the
rotor 24B of the electric motor 32 is mechanically locked up not to
change the rotational relationship between the upper steering shaft
22 and the lower steering shaft 26.
The power steering device 16 may be an oil-hydraulic type power
steering device or an electrically driven power steering device.
However, it is desirable that an electric type power steering
device is used with a ball-screw type motion conversion mechanism
for converting the rotation of an electric motor to a reciprocal
movement of the rack bar 18 so as to decrease the reaction torque
transmitted from the steering angle varying device 24 to the
steering wheel 14.
The braking forces to be applied to the respective vehicle wheels
are controlled by the pressures Pi (i=fl, fr, rl and rr) generated
in wheel cylinders 40FL, 40FR, 40RL and 40RR by an oil-hydraulic
circuit 38. Although not shown in the figure, the oil hydraulic
circuit 38 includes an oil reservoir, an oil pump and various
valves so as to control the pressures in the respective wheel
cylinders normally according to the pressure in a master cylinder
44 of a braking device 36 operated according to a depression of a
brake pedal 42 by the driver, but the oil hydraulic circuit 38 is
also controlled by the electric control device 34 as described in
detail hereinbelow.
In the shown embodiment, a steering angle sensor 50 is provided to
detect the rotational angle of the upper steering shaft 22 as a
steering angle .theta., and a rotational angle sensor 52 is
provided to detect the relative rotation between the housing 24A
and the rotor 24B of the steering angle varying device 24 as a
relative rotation angle .theta.re. The outputs of these sensors are
supplied to the electric control device 34.
The electric control device 34 is supplied with a signal indicating
lateral acceleration Gy of the vehicle from a lateral acceleration
sensor 54, a signal indicating yaw rate .gamma. of the vehicle from
a yaw rate sensor 56, signals indicating wheel speeds Vwi (i=fl,
fr, rl and rr) of the respective vehicle wheels from wheel speed
sensors 58FL-58RR, signals indicating the braking pressures Pi in
the respective wheel cylinders from pressure sensors 60FL-60RR,
signals indicating throttle opening .phi. and engine rotation speed
Ne from an engine control device 62, and other signals.
Although not shown in detail in FIG. 1, the electric control device
34 has a steering control portion for controlling the steering
angle varying device 24, a braking control portion for controlling
the braking forces at the respective vehicle wheels, and a
stability control portion for controlling the running stability of
the vehicle, constructed by a micro computer including CPU, ROM,
RAM, input/output ports and a bilateral common bus interconnecting
these elements. The steering angle sensor 50, the rotational angle
sensor 52, the lateral acceleration sensor 54 and the yaw rate
sensor 56 detect the steering angle .theta., the relative rotation
angle .theta.re, the lateral acceleration Gy and the yaw rate
.gamma. to be positive when the vehicle is steered toward
leftward.
As described hereinunder, the electric control device 34 estimates
the vehicle speed V normally based upon the wheel speeds Vwi of the
respective vehicle wheels, computes the steering gear ratio Rg to
accomplish a predetermined steering performance based upon the
vehicle speed V, computes a temporary target steering angle
.delta.st based upon the steering angle .theta. indicating the
steering operation by the driver and the steering gear ratio Rg,
and computes a target yaw rate .gamma.t of the vehicle based upon
the temporary target steering angle .delta.st and the vehicle speed
V.
Then the electric control device 34 computes a difference
.DELTA..gamma. between the target yaw rate .gamma.t and the actual
yaw rate .gamma. of the vehicle detected by the yaw rate sensor 54,
and when the yaw rate difference .DELTA..gamma. is not larger than
a standard value .gamma.o (positive constant), controls the
steering angle varying device 24 so that the steering angle of the
front left and front right vehicle wheels becomes the temporary
target steering angle .delta.st, whereby the front left and front
right vehicle wheels 10FL and 10FR are steered according to the
steering operation of the driver in a predetermined steering
performance.
The electric control device 34 computes a spin quantity SS
indicating a spinning tendency of the vehicle and a drift out
quantity DS indicating a drifting out tendency of the vehicle based
upon a running condition quantity of the vehicle such as the
lateral acceleration Gy of the vehicle, computes target braking
pressures Pti (i=fl, fr, rl and rr) of the respective vehicle
wheels for stabilizing the vehicle stability against the spin
quantity SS and the drift out quantity DS, and executes a stability
control to control the braking pressures Pi of the respective
vehicle wheels to be the target braking pressures Pti so that
thereby the vehicle behavior is stabilized.
Further, the electric control device 34 computes the vehicle speed
Vb and the braking slips SBi (i=fl, fr, rl and rr) of the
respective vehicle wheels according to a manner known in this art,
and when a condition for starting an anti-skid control is detected
by the braking slips SBi becoming larger than certain threshold
values for starting the anti-skid control (ABS), executes the
anti-skid control by controlling the braking pressures Pi of the
respective vehicle wheels until a condition for ending the
anti-skid control is detected by the braking slips decreasing below
predetermined values.
The electric control device 34 computes the vehicle speed Vb and
the acceleration slips SAi (i=fl, fr, rl and rr) based upon the
wheel speeds Vwi of the respective vehicle wheels in a manner known
in this art, and when a condition for starting the traction control
is detected by the acceleration slips SAi becoming larger than
certain threshold values for starting the traction control (TRC
control), executes the traction control until a condition for
ending the traction control is detected by the acceleration slips
decreasing below determined values.
The above-mentioned the spin/driftout suppress control, anti-skid
control, and traction control do not form the gist of the present
invention, and these controls may be executed according to any
manner known in this art.
The electric control device 34 estimates the longitudinal forces
Fxi of the respective vehicle wheels when the anti-skid control or
the traction control is executed, computes the yaw moment Mf due to
a difference in the longitudinal forces applied to the left and
right vehicle wheels, and computes a first steering angle
compensation amount .DELTA..delta.ct for the front steered vehicle
wheels for applying a counter yaw moment Mc to the vehicle for
canceling the yaw moment induced by the difference in the
longitudinal forces of the vehicle wheels at the left and right
sides of the vehicle.
The electric control device 34 computes a yaw rate difference
.DELTA..gamma. as a difference between the actual yaw rate .gamma.
and the target yaw rate .gamma.t of the vehicle, computes a yaw
angle difference .DELTA..beta. of the vehicle as a difference
between the actual yawing direction and the yawing direction
intended by the driver of the vehicle by integrating the yaw rate
difference .DELTA..gamma., computes a yaw angle .beta.t of the
vehicle for preventing the change of the yawing direction of the
vehicle by the front left and front right vehicle wheels being
steered for the first steering angle compensation amount
.DELTA..delta.ct, computes a second steering angle compensation
amount .DELTA..delta.yt for the front steered vehicle wheels to
decrease the yaw angle difference .DELTA..beta. to 0 for
accomplishing the yaw angle .beta.t based upon the yaw angle
difference .DELTA..beta. and the target yaw angle .beta.t, and
computes a final target steering angle .delta.t for the front
steered vehicle wheels as a sum of the temporary target steering
angle .delta.st, the first steering angle compensation amount
.DELTA..delta.ct and the second steering angle compensation amount
.DELTA..delta.yt. Then, the electric control device 34 controls the
steering angle varying device 24 so that the steering angle of the
front left and front right vehicle wheels becomes the final target
steering angle .delta.t.
FIG. 2 is a flowchart showing the above-mentioned control
operations. The control according to the flowchart of FIG. 2 is
started by a closing of an ignition switch not shown in the figure
and cyclically repeated at a predetermined time interval.
In step 10, the signals indicating the steering angle .theta. and
others are read in, and then in step 20, the vehicle speed V is
estimated based upon the wheel speeds Vwi of the respective vehicle
wheels, and the steering gear ratio Rg is computed based upon the
vehicle speed by referring to a map such as shown in FIG. 3. Then
the temporary target steering angle .delta.st for the front left
and front right vehicle wheels is computed according to the
following formula 19. .delta.st=.theta./Rg (19)
The temporary target steering angle .delta.st is a controlled
steering angle which is a sum of the steering angle .delta.w
(=.theta./Rgo) based upon a standard steering gear ratio Rgo to
correspond to the steering operation by the driver and an
additional steering angle .delta.c for obtaining a predetermined
steering performance. The predetermined steering performance itself
is not concerned with the essence of the present invention, and may
be determined in any manner known in this art, as, for example,
varied according to the steering speed.
In step 30, a standard yaw rate .gamma.e is computed according to
the following formula 20 based upon the wheel base H, a stability
factor Kh, the vehicle speed V and the temporary target steering
angle .delta.st. The standard yaw rate .gamma.e may be computed to
incorporate the lateral acceleration Gy of the vehicle in view of
the dynamic performance of the yaw rate.
.gamma.e=V.delta.st/(1+KhV.sup.2)H (20)
Then, the target yaw rate .gamma.t of the vehicle is computed
according to the following formula 21, wherein T is a time constant
and s is the Laplace operator. .gamma.t=.gamma.e/(1+Ts) (21)
In step 40, a yaw rate difference .DELTA..gamma. is computed as a
difference between the target yaw rate .gamma.t and the actual yaw
rate .gamma. of the vehicle. Then it is judged if the absolute
value of the yaw rate difference .DELTA..gamma. is equal to or
larger than a standard value .gamma.o (positive constant). When the
answer is yes, the control proceeds to step 50, whereas when the
answer is no, the control proceeds to step 90.
In step 50, a yaw angle difference .DELTA..beta. of the vehicle is
computed as a difference between the actual direction of the
vehicle and that intended by the driver by, for example, the yaw
rate difference .DELTA..gamma. being integrated.
In step 60, it is judged if a spin suppress control or a drift out
suppress control is being executed. When the answer is no, the
control proceeds to step 70, whereas when the answer is yes, the
control proceeds to step 90.
In step 70, it is judged if an anti-skid control is being executed.
When the answer is no, the control proceeds to step 80, whereas
when the answer is yes, the control proceeds to step 100.
In step 80, it is judged if a traction control is being executed
for the rear left or rear right vehicle wheel serving as the
driving vehicle wheels. When the answer is yes, the control
proceeds to step 100, whereas when the answer is no, the control
proceeds to step 90.
In step 90, the temporary target steering angle .delta.st is set
for a target steering angle .delta.t for the front left and front
right vehicle wheels.
In step 100, by denoting the inertial moments of the respective
vehicle wheels as Ji, rotational angular accelerations of the
respective vehicle wheels as Vwdi, effective radius of the vehicle
wheels as R, and sums of braking torques Tbi of the respective
vehicle wheels (negative values) and driving torques Tti of the
respective vehicle wheels (positive values) as Txi, the
longitudinal forces (driving/braking forces) of the respective
vehicle wheels Fxi (i=fl, fr, rl and rr) are computed according to
the following formula 22. JiVwdi=RFxi+Txi Fxi=(JiVwdi-Txi)/R
(22)
The rotational angular accelerations Vwdi of the respective vehicle
wheels are computed as a differentiation of each of the wheel
speeds Vwi. The braking torques Tbi are computed based upon the
master cylinder pressure Pm detected by a pressure sensor not shown
in the figure and a pressure-braking torque conversion factor
determined according to the design of the braking device 36. The
driving torques Tdi are computed based upon the engine driving
torque Te and a factor determined according to the design of the
driving system, wherein the engine driving torque Te is computed
based upon the throttle opening .phi. and the engine rotation speed
Ne input from the engine control device 62. The braking torques Tbi
and the driving torques Tdi may be directly detected by torque
sensors.
In step 110, by denoting the tread of the vehicle as T, the yaw
moment Mf due to the difference in the longitudinal forces acting
at the left and right vehicle wheels is computed based upon the
longitudinal forces Fxi of the respective vehicle wheels according
to the following formula 23. Mf={(Fxfr+Fxr)-(Fxfl+Fxrl)}T/2
(23)
In step 120, a first steering angle compensation amount
.DELTA..delta.ct for the front left and front right vehicle wheels
for generating a counter yaw moment by the steering of the front
left and front right vehicle wheels for canceling the yaw moment Mf
is computed according to the following formula 24 corresponding to
the above-mentioned formula 13. The first steering angle
compensation amount .DELTA..delta.ct may be computed according to a
formula corresponding to the above-mentioned formula 12.
.DELTA..delta..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times.
##EQU00009##
In the calculation by the formula 24 or the formula corresponding
to the above-mentioned formula 12, the cornering power Cpf of the
front vehicle wheels and the cornering power Cpr of the rear
vehicle wheels should be modified according to the slip ratios of
the respective vehicle wheels.
In step 130, a yaw angle .beta.t of the vehicle for canceling the
lateral forces generated by the front left and front right vehicle
wheels due to the steering thereof for the first steering angle
compensation amount .DELTA..delta.ct by a lateral force of the rear
left and rear right vehicle wheels is computed according to the
above-mentioned formula 18.
In step 140, the second steering angle compensation amount
.DELTA..delta.yt for the front left and front right vehicle wheels
for making the yaw angle difference .DELTA..beta. to 0 by attaining
the yaw angle .beta.t is computed based upon the yaw angle
difference .DELTA..beta. and the yaw angle .beta.t according to the
following formula 25, wherein Kx is a predetermined factor.
.DELTA..delta.yt=Kx(.DELTA..beta.+.beta.t) (25)
In step 150, a final target steering angle .delta.t for the front
left and front right vehicle wheels is computed as a sum of the
temporary target steering angle .delta.st, the first steering angle
compensation amount .DELTA..delta.ct and the second steering angle
compensation amount .delta..beta.yt. Then in step 160, the steering
angle of the front left and front right vehicle wheels is
controlled by the steering angle varying device 24 so that the
steering angle of the front left and front right vehicle wheels
becomes the final target steering angle .delta.t.
Although the present invention has been described in detail with
respect to a particular embodiment thereof, it will be apparent for
those skilled in the art that various modifications are possible
within the scope of the present invention.
For example, although in the shown embodiment, the yaw angle
.beta.t of the vehicle is a yaw angle which is the same in the
magnitude as the slip angle .beta. of the vehicle and opposite
thereto in the lateral direction, the yaw angle .beta.t may be an
angle which is smaller than the slip angle .beta. in the magnitude
and opposite in the lateral direction.
Further, although in the shown embodiment, the counter yaw moment
applied to the vehicle for canceling the yaw moment Mf due to the
difference in the longitudinal forces is the same in the magnitude
as the yaw moment Mf and opposite in the direction, the counter yaw
moment may be smaller than the yaw moment Mf in the magnitude.
Further, although in the shown embodiment, steps 100-150 are not
executed when a spin or driftout suppress control is being
executed, the control of steps 100-150 may be executed even when a
spin or driftout control is being executed with a control for
sharing the driving/braking force between the left and right
vehicle wheels.
Further, although in the shown embodiment, the vehicle is a rear
driven vehicle, the present invention may be applied to a four
wheel drive vehicle, or a wheel-in-motor type vehicle in which the
respective vehicle wheels are driven by respective driving
motors.
* * * * *